This invention relates to a method and apparatus for packaging by forming adhesively sealed packages. More particularly, this invention relates to a method and apparatus for automatically forming packages using two webs of plastic material and a hot melt adhesive applied along a seal-line.
U.S. Pat. No. 3,061,984 illustrates packaging apparatus of known type for making hermetically-sealed vacuum packages using heat sealing to weld together two webs of plastic packaging material to form the packages. In carrying out this method of package formation, the inner surfaces of each of the two webs of packaging material typically will be of the same composition or otherwise compatible to be able to be welded by heating.
It later became common practice in vacuum-packaging sliced luncheon meats and the like to use as one of the two webs plastic material of much heavier caliber and of greater stiffness, e.g. as shown and described in U.S. Pat. No. 3,545,163. Such packages were somewhat difficult to open by hand Accordingly, sealants on the inner surfaces of the two webs forming the package were formulated so that the hermetic seal could be opened with a relatively low force by peeling the two package components apart.
The heat sealing methods described above have come into widespread use. The materials used in such packages typically are multilayered, often incorporating an inner layer whose only function is to provide the required heat sealing capability. Web back-forming, such as described in U.S. Pat. No. 3,545,163, has been employed with both flexible webs (11/2 to approximately 5 mils thick) and semi-rigid webs (having a thickness in the range of 7 to 18 mils). Web backforming is capable of providing a package with a platform type appearance as illustrated in FIG. 4 of U.S. Pat. No. 3,229,810. Normally packages fabricated using two webs of flexible material do not include a back-formed component.
The prior art packages which have been described have for the most part been sealed by heating and welding the layers together or by interposing a plasticizer of pressure sensitive adhesive. Hot melt adhesives for sealing vacuum packages have found limited use, even though they offer certain advantages in reducing the cost of the packaging materials required, as well as in providing the ability to reclose and re-seal the packages after the initial opening.
Hot melt adhesives have been applied by several methods. One method includes applying the hot melted adhesive to one of two package components comprising a partially processed semi-rigid component with the application being made at a time well in advance of the actual packaging operation. In this method of packaging, the hot melt adhesive must be allowed to cool and then be reactivated at the time of sealing by using heat and pressure. This method is more expensive to carry out than a simple in-line adhesive application method, and has been practical only when used on semi-rigid material.
Another method for applying hot melt adhesive is through use of a rotogravure printer. This method has not been commercially satisfactory for packaging machines because of operational problems as well as the difficulty of printing on areas depressed below a web line or on three dimensional areas. Furthermore, since the process inherently involves continuous motion in performing the printing operation, problems exist in attempting to use the method on an intermittent-motion machine.
A further method, which has had limited commercial use, is the application of the adhesive by a hot melt adhesive applicator gun which may be stationary while the packages move past the gun; alternatively, the guns may be traversed over the desired patterns of the package, or a combination of these two approaches may be used. The traversing gun has represented the best technique since it requires only one gun per package being traversed. However, the guns and their carriers must be started and stopped in their traversing path once for every pattern being laid down. Accordingly, the acceleration as well as the stopping of the moving guns places an upper practical limit on the number of packages which can be processed in a given time period. Furthermore, the traversing equipment is complicated and expensive to manufacture and maintain.
It also is difficult to apply a thin line of hot melt adhesives without a break, skip or excessive overlap at the high traversing speeds required to first accelerate or begin application, and then to shut off and decelerate the applicator head all within one machine cycle. Due to the bulk of the apparatus in previous designs, it has been impractical to apply the hot melt adhesive immediately prior to sealing thereby requiring extensive reactivation of the adhesive with heat and pressure which makes the bonds between the package components less than optimum because of elapsed time, particularly in a typical refrigerated work room.